Electricity meters are devices that, among other things, measure the consumption of electricity by a residence, factory, commercial establishment or other such facility. Electricity meters may also be employed to measure electricity flow between different electrical power generation or delivery systems. Electrical utility service providers employ electricity meters to track customer usage of electrical power or energy. Utilities track customer usage for many purposes, including billing and tracking demand.
At present electricity meters often include digital circuitry that performs many of the energy-related calculations. Examples of such meters are shown and discussed in U.S. Pat. No. 5,544,089, U.S. Pat. No. 5,933,004, and U.S. Pat. No. 6,112,158, which are incorporated herein by reference. Such meters, referred to generally herein as electronic meters, typically include analog sensor circuitry and digital measurement circuitry.
The analog sensor circuitry includes one or more sensor devices that sense or detect current and/or voltage levels on the power lines of the system being measured. The sensor devices generate analog measurement signals that are representative of the detected current and voltage waveforms actually present on the power lines. The analog sensor circuitry typically provides the signals to analog to digital conversion circuitry which generates digital versions of the measurement signals.
In an electronic meter, the measurement circuit typically performs energy-related calculations on the digitized measurement signals. In general, the measurement circuit will perform, among other things, an energy or watt-hour calculation.
To this end, measurement circuit multiplies the digitized voltage measurement signal by the digitized current measurement signal. In particular, the digitized measurement signals consist of sampled voltage measurement values and sampled current measurement values. By multiplying the individual voltage samples by the individual current samples and summing the resulting products over time, energy consumption values are obtained. In particular, in a single phase system, the energy consumption measurement may be given by the following equation:WH=ΣV(n)*I(n)*Tn; for n=1 to N,Where WH is equal to energy consumption (e.g. watt-hours), Tn is the sample period of n, V(n) is the nth voltage sample, and I(n) is the nth current sample that is sampled contemporaneously with V(n). Where WH is equal to energy consumption (e.g. watt-hours), V(n) is the nth voltage sample, and I(n) is the nth current sample that is sampled contemporaneously with V(n).
The measurement circuit thereafter typically displays or communicates the calculated watt-hour data to a centralized computer or the like. The measurement circuit may also perform intermediate calculations to convert the WH data into units of measurement typically used in the metering industry. The measurement circuit may further perform various data tracking operations and/or control relays or other external devices responsive to the measured data. It is noted that the measurement circuit may calculate other energy consumption measurement values, such as RMS current, RMS voltage, reactive energy, apparent energy, or various power values. Such measurement values, as well as other energy or power related values generated from the measurement signals, are referred to generically herein as energy consumption data.
One important element of electricity meters, including electronic meters, is metering accuracy. Metering accuracy is important because inaccurate metering can result in substantial amounts of lost revenue. Moreover, inaccurate metering can also undesirably result in overcharging of customers.
The common sources of metering inaccuracies, or error sources in a meter, include the sensor devices in the sensor circuitry. In particular, sensor devices can produce error in both the magnitude of the measurement signals and the phase of the measurement signal waveform. For example, current transformers, which generate current measurement signals, often introduce significant magnitude and phase error into the current measurement signal. Such errors propagate through to the calculated energy consumption data.
To reduce the errors due to the sensor devices, electronic meters are typically calibrated. In particular, it is known to introduce a calibration factor into the energy calculation to compensate for current transformer error. The calibration factor is typically determined by subjecting the meter to a calibration procedure in the factory. The calibration procedure involves attaching the meter to measure a known amount of energy. The calibration factor is derived from the difference in the known amount of energy delivered and the amount of energy actually registered by the meter.
While such calibration procedures may reduce the inaccuracy due to error sources within the meter, factory calibration procedures are inadequate for addressing error sources external to the meter. One external error source is an instrument transformer. An instrument transformer is a transformer that is connected between the meter and the power lines. Instrument transformers are used to scale down the voltage and/or current that is actually delivered to and measured by the meter. Instrument transformers are uncommon in single residence applications, but are relatively common in larger, higher voltage and current systems, such as those used for large industrial or commercial establishments.
Instrument transformers, like the current transformers within the meter, often exhibit notable phase error and magnitude or ratio error. Indeed, it is common in the instrument transformer industry to provide a visual indication of the phase error and ratio error data on the instrument transformer itself. Such data are referred to as error ratings, including phase error ratings and ratio error ratings.
Because instrument transformers are external to the meter, factory calibration of the meter to compensate for the instrument transformer ratio and phase error is impracticable. In particular, instrument transformers are often installed independently of the meter, and indeed may be supplied from a different supplier than the electricity meter supplier. Thus, current calibration procedures cannot be used to calibrate the meter for the error caused by the external instrument transformers.
Errors due to instrument transformers are not insignificant. Accordingly, inaccurate metering and billing due to ratio and/or phase errors in instrument transformers can cost utilities and customers significant amounts of money. As a result, there is a need for some method (and apparatus) that corrects or compensates for errors caused by instrument transformers, or any transformer located external to the meter, to ensure relatively accurate metering.